Catalytic burners are known for generating and transferring heat to a thermally conductive heat sink, such as the heater acceptor of a Stirling engine. For the purposes of this invention, the terms “burner” and “combustor” are used interchangeably and are meant to imply an apparatus that under operable conditions achieves full combustion of a fuel with an oxidant primarily to form deep oxidation products, namely, carbon dioxide and water. Catalytic burners operate by contacting the fuel with the oxidant in the presence of a combustion catalyst capable of facilitating conversion of the fuel to carbon dioxide and water. As generally understood, the “catalyst” is a material or chemical composition that facilitates the combustion essentially without being consumed in the combustion. Catalytic combustion tends to be flameless. Catalytic burners start up in flame mode using an ignition device. Upon reaching a temperature sufficient for self-sustaining combustion (“light-off”), the ignition device is typically de-energized. The flame is then extinguished and catalytic combustion continues flamelessly.
Certain combustors, such as those described in application publication no. US 2013/0266903, corresponding to PCT application publication WO 2012/106048, may be operated in flame mode, or operated in catalytic flameless mode, or operated simultaneously in both flame and flameless modes, as desired. Such dual functional combustors are referenced herein as “hybrid burners.” As disclosed in US 2013/0266903, a single fuel inlet and a single oxidant inlet are aligned around a longitudinal axis of the burner so as to deliver a fuel-oxidant reaction mixture into the combustion chamber. The combustion reaction is ignited in flame mode with resultant heat of reaction, which heats a catalyst positioned downstream of the fuel and oxidant inlets. At an appropriate temperature, the catalyst lights-off resulting in catalytic combustion. Heat of reaction is transmitted along the longitudinal axis into a heat sink. The hybrid burner may continue in simultaneous flame and flameless modes; or alternatively, the flame can be extinguished for operation solely in catalytic flameless mode.
The prior art, for example U.S. Pat. No. 8,479,508, discloses another catalytic combustor comprising a combustion chamber, one fuel inlet, one or two oxidant inlets, a combustion catalyst positioned within the combustion chamber, a heat acceptor surface positioned downstream of the catalyst, a heat spreader positioned in between and in direct contact with the combustion catalyst and the heat acceptor surface, and an outlet for exhausting combustion gases. The catalyst is taught to be constructed from an ultra-short-channel-length substrate, preferably a metal mesh having one or more noble metals deposited thereon. The fuel and oxidant inlets are positioned along a central longitudinal axis; while the combustion catalyst, heat spreader, and heat acceptor surface are layered and located downstream of the inlets in a radial plane within the combustion chamber. Heat is transferred in a longitudinal direction down the burner.
U.S. Pat. No. 8,387,380 discloses another flameless catalytic burner comprising a combustion chamber, one fuel inlet, one oxidant inlet, a combustion catalyst positioned within the chamber directly contacting a heat acceptor, and an outlet for exhausting combustion gases. Fuel and oxidant inlets are positioned along a central longitudinal axis; while the combustion catalyst is wrapped in a ring shape around a cylindrical heat sink. Combustion gases flow in and out of the burner in a longitudinal direction; while heat transfer is directed radially from the catalyst into the heat sink.
Notably, the catalytic combustors disclosed in the prior art use the interior volume of the combustor upstream of the catalyst to introduce and mix the fuel and oxidant. Introduction of fuel and oxidant occur near the center longitudinal axis of the combustor. The fuel and oxidant will mix and expand in a cone-shaped volume; and then flow longitudinally through the catalyst, which is positioned in a flat radial plane as in U.S. Pat. No. 8,479,508, or in a ring shape as in U.S. Pat. No. 8,387,380. An amount of interior volume outside the edges of the cone-shaped region is unused “dead space”. For smaller burner designs (100-500 Watts thermal input), the unused dead space comprises an inconsequential portion of the burner volume and is typically not practically useful for other purposes. For larger burners (>500 Watts thermal input), the unused dead space is wasted.
We have recognized a need for a catalytic combustor that substantially eliminates wasted or “dead space” and that, preferably, renders useful essentially all of the volume within the space enveloped by the combustor. It would be desirable to design the catalytic combustor such that it could accommodate secondary utility components without sacrificing combustion efficiency. As examples, we have recognized that it would be desirable to design a catalytic combustor that could accommodate within the “dead space” a Stirling engine, or an auxiliary system component, such as a water coil or a heat recuperator, or another source of heat, such as a source of solar or geothermal heat.